libata-acpi: add missing inlines in libata.h

Adds inline for ata_acpi_unregister/ata_acpi_bind/ata_acpi_unbind
in drivers/ata/libata.h for !CONFIG_ATA_ACPI to fix below warnings.

warning: 'ata_acpi_unregister' defined but not used [-Wunused-function]
warning: 'ata_acpi_bind' defined but not used [-Wunused-function]
warning: 'ata_acpi_unbind' defined but not used [-Wunused-function]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>